1. Field of the Invention
The present invention relates to a sizing agent for paper, which is useful for sizing paper or paper board (hereinafter paper and paper board will be generally referred to simply as "paper") and paper sized with such a sizing agent.
2. Discussion of Background
Heretofore, various sizing agents have been used To improve various strength and printability of paper or the appearance of printed matters. Among such sizing agents, those using a polyvinyl alcohol, a starch or both of them, are typical.
However, paper sized with a starch has a drawback such that when it is subjected to printing, paper powder is likely to form, and such paper powder tends to deteriorate the appearance of the printed matter, or such paper powder is likely to accumulate between printing rollers, thus leading to a trouble of e.g. rupture of the paper. Further, paper sized with a starch tends to have inadequate strength, whereby defects such as picking and blistering are likely to occur at the printed surface, and such tendency is particularly remarkable when the printing speed is high. On the other hand, paper sized with a polyvinyl alcohol has a drawback such that it shows tackiness in a state wet with water, although it exhibits excellent properties with no substantial formation of paper powder or with no substantial deterioration of the appearance due to damage of the printed surface given by high speed printing. This drawback causes, for example, a printing trouble such that when the paper sized with a polyvinyl alcohol is subjected to offset printing, if the paper is wetted upon contact with damping water, tackiness will be created, whereby a trouble such as adhesion of the paper to itself or adhesion of the paper to the printing rollers, is likely to result. If such a trouble occurs during printing, the printing operation will have to be stopped, thus leading to a substantial loss in the productivity and a loss of materials, and further the machine is likely to be damaged.
Japanese Unexamined Patent Publication No. 41611/1974 discloses a sizing agent having a non-ionic surfactant incorporated to a polyvinyl alcohol and having a third component such as urea, a urea derivative, thiourea, a thiourea derivative or a guanidine derivative incorporated. However, this sizing agent requires a third component such as urea, as an essential component. Besides, the effect for reducing the tackiness of the sized paper in a wet state, is still inadequate.
Further, Japanese Unexamined Patent Publication No. 295693/1993 discloses a surface sizing agent for paper comprising a polyvinyl alcohol resin and polyethylene glycol as essential components. However, this sizing agent has a problem with respect to printability.
Reflecting a trend in recent years for multi-color printing or weight reduction of medium or low quality paper such as paper for newspapers, which is subjected to high speed printing in a large quantity, it has become common to size such medium or low quality paper, and it has become common to employ an offset printing system as a printing system for such sized paper. Therefore, it has been desired to develop a sizing agent for paper which presents low tackiness even when the sized paper is in a wet state and which is excellent in printability.